Our primary objective is to determine the molecular events that lead to the selective transcription of DNA and subsequent processing and translation of messenger RNA (mRNA). Poxviruses provide a unique and important system for such studies since the enzymes needed for transcription and mRNA modification are packaged within the core of infectious virus particles. A single enzyme complex containing three activities - RNA triphosphatase, RNA guanylyltransferase, and RNA (guanine-7-)methyltransferase - has been purified from vaccinia virus. Acting consecutively, the individual activities in the complex are capable of modifying the 5'-end of nascent RNA molecules to form a cap structure in vitro. Extracts from uninfected human cells were shown to carry out a similar set of reactions, however, three separate enzymes were found instead of a single complex.